In Situ XAS Investigation of Transformation of Co Monolayer on Carbon-Supported Platinum Clusters Underpotential Control

Development of photochemical and electrochemical cells for operando X-ray absorption spectroscopy during photocatalytic and electrocatalytic reactions

Photochemical and electrochemical reactions are highly relevant processes for (i) transforming chemicals (e.g. photoreduction of isopropanol to acetone, electrochemical hydrogenation of benzaldehyde to benzyl alcohol, etc.), and (ii) sustainable energy production (e.g. photoreduction of CO2 to methanol, electrocatalytic H2 evolution reaction). It is therefore of importance to monitor the structural changes and to understand the properties of active sites under photocatalytic and electrocatalytic reaction conditions. Operando X-ray absorption spectroscopy (XAS) provides the means to investigate the nature of active sites under realistic reaction conditions. In this contribution, we describe the successful development of photochemical and electrochemical cells for operando XAS measurements during photocatalytic and electrocatalytic reactions. We have used the operando photochemical cell to monitor the formation of Pt nanoparticles on graphitic carbon nitride nanosheets (g-C3N4-ns) via photodeposition under visible light illumination and observed the formation of highly dispersed Pt nanoparticles with an estimated size of ∼2.5 nm and >60% dispersion. We have also tested this cell to follow the oxidation state of Pt in Pt/TiO2 and Pt/g-C3N4-ns during H2 evolution reaction (HER). We observed that Pt predominantly existed as metallic (reduced) Pt0 species under HER conditions, and that PtOx species were partially reduced from PtIV to Pt0 upon illumination with UV or visible light. The rates of H2 evolution obtained in the photochemical cell (12.1 mmol g-1 h-1 on Pt/TiO2 and 1.01 mmol g-1 h-1 on Pt/g-C3N4-ns) were comparable to that obtained in a standard top-irradiated photoreactor (16.6 mmol g-1 h-1 on Pt/TiO2 and 1.76 mmol g-1 h-1 on Pt/g-C3N4-ns). The operando electrochemical cell was successfully tested to monitor the changes in the structure and oxidation state of Pd in Pd/C electrocatalyst during electrocatalytic hydrogenation (ECH) of benzaldehyde. It was demonstrated that Pd in Pd/C was present in a partially reduced state (∼80% Pd0 and ∼20% PdII) and Pd nanoparticles did not degrade upon the application of an external potential under ECH reaction conditions.

Low cost facile synthesis of large-area cobalt hydroxide nanorods with remarkable pseudocapacitance

Large-area Co(OH)2-based supercapacitor electrodes composed of nanotube arrays grown on a 3D nickel-foam (CONTA) electrode and sucker-like nanoporous films grown on a 3D nickel-foam (COSNP) electrode were prepared with a facile electrochemical method for applications in energy storage. These nanoporous Co(OH)2 electrodes were fabricated with the codeposition of Cu/Ni film on the nickel foam, then etching of Cu from the Cu/Ni layer to form Ni nanotube arrays and sucker-like Ni nanoporous layers, and further cathodic deposition of Co(OH)2 on the prepared nanoporous Ni substrates. The CONTA and COSNP electrodes exhibited specific capacitances of 2500 and 2900 F/g in a voltage range of 0.65 V (capacitance of the substrates deducted from the total) at 1 A/g in a three electrode cell, respectively. The COSNP electrode demonstrated an excellent supercapacitive performance with specific capacitances 1100 F/g at 1 A/g and 850 F/g at 20 A/g in a voltage range of 1.2 V in a two electrode cell. The remarkable performance of COSNP electrodes correlated with a large conversion of the Co oxidation state during the charge/discharge cycling were examined by in situ X-ray absorption near edge structure (XANES).

Tunable properties of PtxFe1-x electrocatalysts and their catalytic activity towards the oxygen reduction reaction

We present the controlled synthesis of bimetallic Pt(x)Fe(1-x) nanoparticles with tunable physical properties and a study of their catalytic activity towards the oxygen reduction reaction (ORR). Composition-induced variations in alloying extent and Pt d-band vacancies in Pt-Fe/C catalysts are systematically investigated. Density functional theoretical calculations are performed in order to realize the electronic effect caused by alloying Pt with Fe. The DFT computational observations revealed that iron donates electrons to platinum, when the Fe 3d and Pt 5d orbitals undergo hybridization. The Pt(x)Fe(1-x) catalysts with various Pt-to-Fe atomic ratios are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammetry (CV), and X-ray absorption spectroscopy (XAS). TEM images indicate that the dispersion of the metal nanoparticles is uniform and the XAS technique provides significant insight on Pt d-band vacancies and the alloying extent of Pt and Fe in Pt(x)Fe(1-x) nanoparticles. Rotating-disk voltammetry of Pt(x)Fe(1-x) nanoparticle catalysts with various Pt : Fe atomic compositions (3 : 1, 1 : 1, and 1 : 3) revealed that the Pt(1)Fe(1)/C nanocatalyst showed a greater enhancement in ORR activity than platinum. The enhanced catalytic activity toward ORR is attributed to the higher alloying extent of platinum and iron as well as the promising electronic structure offered by the lower unfilled Pt d states in Pt(x)Fe(1-x) nanoparticles when compared to pure Pt.

Probing compositional variation within hybrid nanostructures

We present a detailed analysis of the structural and magnetic properties of solution-grown PtCo-CdS hybrid structures in comparison to similar free-standing PtCo alloy nanoparticles. X-ray absorption spectroscopy is utilized as a sensitive probe for identifying subtle differences in the structure of the hybrid materials. We found that the growth of bimetallic tips on a CdS nanorod substrate leads to a more complex nanoparticle structure composed of a PtCo alloy core and thin CoO shell. The core-shell architecture is an unexpected consequence of the different nanoparticle growth mechanism on the nanorod tip, as compared to free growth in solution. Magnetic measurements indicate that the PtCo-CdS hybrid structures are superparamagnetic despite the presence of a CoO shell. The use of X-ray spectroscopic techniques to detect minute differences in atomic structure and bonding in complex nanosystems makes it possible to better understand and predict catalytic or magnetic properties for nanoscale bimetallic hybrid materials.